Morphometric Analysis of Fall Armyworm (Spodoptera frugiperda) Larvae Reared on Different Diets

Fall armyworm or Spodoptera frugiperda is one of the main pests of corn. It can caused up to 28.08% yield loss. Pest control efforts with broad-spectrum insecticides can cause negative impacts that disrupt ecosystems, non-target biotic factors and the surrounding environment. Beauveria bassiana is an alternative biological control agent that is target-specific. This study was conducted to determine toxic and antifeedant activities of B. bassiana application on third to fifth-instar S. frugiperda larvae. B. bassiana isolates were propagated using corn flakes media and resulted in colonies with conidia density of 1.1×108conidia.g-1. This study used a completely randomized design with 5 treatments and 5 replications. Treatments consisted of P1 namely B. bassiana propagation with a concentration of 60 g.L-1, P2 = 70 g.L-1, and P3 = 80 g.L-1. P0 was a negative control (untreated with B. bassiana) and P4 was a positive control (diazinon with concentrations of 2 ml.L-1). The variables observed were mortality and weight of the leftover feed. The results showed that the formulation of B. bassiana with concentrations of 80 g.L-1 was able to infect armyworms with a mortality percentage of 53.5% which was statistically similar as the effect of chemical insecticides. In addition, inhibition of feeding activity due to treatment could prevent leaf damage up to 68.21%.

Induction of detoxification enzymes by triazine herbicides in the fall armyworm, Spodoptera frugiperda (J.E. Smith)

Entomopathogenic fungi have been considered potential biological control agents against the fall armyworm Spodoptera frugiperda (J. E. Smith), the world's most important pest of maize. In this study, we evaluated the natural infection, molecular characteristics, and biological activity of Metarhizium rileyi (Farlow) isolated from S. frugiperda larvae of this insect, collected from maize crops in five Mexican locations. Natural infection ranged from 23% to 90% across all locations analyzed. Twenty-four isolates were evaluated on S. frugiperda second instars at a concentration of 1.0 × 108 conidia/mL, causing 70% to 98.7% mortality and 60.5% to 98.7% sporulation. Isolates T9-21, Z30-21, PP48-21, and L8-22 were selected to determine their phylogenetic relationships by β-tubulin gene analysis and to compare median lethal concentration (CL50), median lethal time (LT50), and larval survival. These isolates were grouped into three clades. The T9-21, PP48-21, and J10-22 isolates were closely related (clade A), but phylogenetically distant from Z30-21 (clade B) and L8-22 (clade C) isolates. These genetic differences were not always reflected in their pathogenicity characteristics since no differences were observed among the LC50 values. Furthermore, isolates T9-21, J10-22, and L8-22 were the fastest to kill S. frugiperda larvae, causing lower survival rates. We conclude that native M. rileyi isolates represent an important alternative for the biocontrol of S. frugiperda.

The fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is an important pest of warm-season turfgrass species, including bermudagrass (Cynodon spp.). Bermudagrass is a popular turfgrass that is widely planted on golf courses, athletic grounds, and ornamental landscapes across the country and throughout the world. Spodoptera frugiperda infestation is often sporadic; however, when it does occur, damage can be severe. Host plant resistance against S. frugiperda can be a valuable tool for reducing or preventing the use of insecticides. Therefore, the objective of this study was to determine resistance against S. frugiperda in a few promising bermudagrasses. Fourteen experimental bermudagrass genotypes plus two control cultivars, ‘Zeon’ zoysiagrass (resistant control) and ‘TifTuf’ bermudagrass (susceptible control), were evaluated against S. frugiperda to determine host plant resistance in the laboratory. The results showed that the resistant control, ‘Zeon’ zoysiagrass, was more resistant than the other genotypes to S. frugiperda larvae. To determine the response of the experimental lines to S. frugiperda as compared with that of the controls, three indices were developed based on survival, development, and overall susceptibility. According to the susceptibility index, ‘13-T-1032’, ‘T-822’, ‘11-T-510’, ‘12-T-192’, ‘11-T-56’, ‘09-T-31’, ‘11-T-483’, and ‘13-T-1067’ were the top-ranked bermudagrasses. Among these, the responses of ‘13-T-1032’, ‘T-822’, ‘11-T-510’, ‘11-T-56’, ‘09-T-31’, and ‘11-T-483’ were comparable to that of ‘TifTuf’, and antibiosis was the underlying mechanism of resistance. Additionally, larval length, head capsule width, and weight were negatively associated with the days of pupation and adult emergence and positively associated with pupal length, thorax width, and weight. These results will help refine future breeding and with investigations of resistance against the fall armyworm.

Corn earworm (Heliothis zea) and fall armyworm (Spodoptera frugiperda) larvae were fed on diets consisting of maize kernels fermented (8 da; 28 C) with individual strains of or domesticated yellow-green aspergilli (e.g., wild isolates = Aspergillus flavus var. flavus, A. flavus var. parasiticus; domesticated, non-aflatoxigenic koji molds = A. flavus var. oryzae, A. flavus var. sojae). Kernels fermented with individual strains of var. sojae were less toxic to larvae than kernels fermented with isolates of the related wild variety parasiticus. Likewise, kernels fermented with isolates of var. flavus were, with one exception (NRRL 6513), more toxic to fall armyworm larvae than kernels fermented with isolates of the related domesticated variety oryzae. As a group, strains of var. oryzae were no less toxic to corn earworm larvae than the var. flavus strains that were tested. Toxic fungal metabolities are believed to be the direct cause of larval mortality because dead larvae were not colonized by fungal hyphae. This is consistent with the hypothesis that wild strains ofA. flavus produce a suite of defensive entomotoxic metabolites having no adaptive value in a koji environment. We suggest that koji molds used in food fermentations be screened by means of insect bioassays as a relevant test of the toxigenic potential of individual domesticated strains.

Plants can have fundamental roles in shaping bacterial communities associated with insect herbivores. For larval lepidopterans (caterpillars), diet has been shown to be a driving force shaping gut microbial communities, where the gut microbiome of insects feeding on different plant species and genotypes can vary in composition and diversity. In this study, we aimed to better understand the roles of plant genotypes, sources of microbiota, and the host gut environment in structuring bacterial communities. We used multiple maize genotypes and fall armyworm (Spodoptera frugiperda) larvae as models to parse these drivers. We performed a series of experiments using axenic larvae that received a mixed microbial community prepared from frass from larvae that consumed field-grown maize. The new larval recipients were then provided different maize genotypes that were gamma-irradiated to minimize bacteria coming from the plant during feeding. For field-collected maize, there were no differences in community structure, but we did observe differences in gut community membership. In the controlled experiment, the microbial inoculation source, plant genotype, and their interactions impacted the membership and structure of gut bacterial communities. Compared to axenic larvae, fall armyworm larvae that received frass inoculum experienced reduced growth. Our results document the role of microbial sources and plant genotypes in contributing to variation in gut bacterial communities in herbivorous larvae. While more research is needed to shed light on the mechanisms driving this variation, these results provide a method for incorporating greater gut bacterial community complexity into laboratory-reared larvae.

Corn earworm (Heliothis zea) and fall armyworm (Spodoptera frugiperda) larvae were fed on diets consisting of maize kernels fermented (8 da; 28 C) with individual strains of “wild” or domesticated yellow-green aspergini (e.g., wild isolates = Aspergillus flavus var. flavus, A. flavus var. parasiticus; domesticated, non-aflatoxigenic koji molds = A. flavus var. oryzae, A. flavus var. sojae). Kernels fermented with individual strains of var. sojae were less toxic to larvae than kernels fermented with isolates of the related wild variety parasiticus. Likewise, kernels fermented with isolates of var. flavus were, with one exception (NRRL 6513), more toxic to fall armyworm larvae than kernels fermented with isolates of the related domesticated variety oryzae. As a group, strains of var. oryzae were no less toxic to corn earworm larvae than the var. flavus strains that were tested. Toxic fungal metabolities are believed to be the direct cause of larval mortality because dead larvae were not colonized by fungal hyphae. This is consistent with the hypothesis that wild strains of A. flavus produce a suite of defensive entomotoxic metabolites having no adaptive value in a koji environment. We suggest that koji molds used in food fermentations be screened by means of insect bioassays as a relevant test of the toxigenic potential of individual domesticated strains.

Although most of the damage caused by lepidopteran insects to plants is caused by the larval stage, chemosensory systems have been investigated much more frequently for lepidopteran adults than for larvae. The fall armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a polyphagous and worldwide pest. To understand the larval chemosensory system in S. frugiperda, we sequenced and assembled the antennae and maxillae transcriptome of larvae in the sixth instar (larval a-m) using the Illumina platform. A total of 30 putative chemosensory receptor genes were identified, and these receptors included 11 odorant receptors (ORs), 4 gustatory receptors (GRs), and 15 ionotropic receptors/ionotropic glutamate receptors (IRs/iGluRs). Phylogeny tests with the candidate receptors and homologs from other insect species revealed some specific genes, including a fructose receptor, a pheromone receptor, IR co-receptors, CO2 receptors, and the OR co-receptor. Comparison of the expression of annotated genes between S. frugiperda adults and larvae (larval a-m) using RT-qPCR showed that most of the annotated OR and GR genes were predominantly expressed in the adult stage, but that 2 ORs and 1 GR were highly expressed in both the adult antennae and the larval a-m. Although most of the tested IR/iGluR genes were mainly expressed in adult antennae, transcripts of 3 iGluRs were significantly more abundant in the larval a-m than in the adult antennae of both sexes. Comparison of the expression levels of larval a-m expressed chemosensory receptors among the first, fourth, and sixth instars revealed that the expression of some of the genes varied significantly among different larval stages. These results increase our understanding of the chemosensory systems of S. frugiperda larvae and provide a basis for future functional studies aimed at the development of novel strategies to manage this pest.

Previous experience on host plants can modify insect feeding behavior. Because insect habituation and induction of preference to host plants are variable across species of plants and insects, it is necessary to investigate each insect-plant interaction to determine whether this phenomenon occurs or not in the system. In this study we investigated the potential occurrence of habituation and induction of preference in fall armyworm (FAW) Spodoptera frugiperda to soybean genotypes. Neonate FAW larvae reared on artificial diet were divided into four treatment groups and fed for one generation with either the resistant soybeans PI 227687 or IAC 100 or the susceptible soybeans BRS Valiosa RR or IGRA RA 626 RR. Biological parameters of FAW were recorded. Eggs obtained from FAW of each genotype group were separated, and the newly hatched larvae were fed on the same genotypes experienced by their parents for additional 8 days. FAW larval preference and leaf area consumed were evaluated in choice feeding assays with the four soybean genotypes within a 24-h period. Genotypes PI 227687 and IAC 100 negatively affected FAW development, demonstrating they are FAW-resistant. FAW larvae exposed to both resistant genotypes consumed more foliage of genotype IGRA RA 626 RR in the choice assays, whereas larvae reared on both susceptible genotypes did not show any preference. From our preliminary study, FAW does not show habituation and induction of preference toward the experienced soybean genotypes. The importance of our findings to host plant resistance and insect-plant biology fields is discussed.

Influence of three insecticides targeting GABA receptor on fall armyworm Spodoptera frugiperda: Analyses from individual, biochemical and molecular levels

A novel volatile deterrent from symbiotic bacteria of entomopathogenic nematodes fortifies field performances of nematodes against fall armyworm larvae

The fall armyworm (FAW) Spodoptera frugiperda (J. E. Smith), native to the Americas, is a new invasive pest that was reported in India for the first time in May 2018. Being polyphagous, FAW can infest several different hosts and increase its population all year round. In this context, the present study was conducted under laboratory conditions to evaluate the biological parameters of FAW on four different hosts, Zea mays (maize), Gossypium hirsutum (cotton), Ricinus communis (castor) and Brassica oleracea var. botrytis (cauliflower), and a semi-synthetic diet. The shortest life cycle of 32.8 ± 0.52 days in males and 34.1 ± 0.43 days in females was observed on maize. Semi-synthetic diet was superior in terms of higher mean fecundity (1324.6 ± 61.21 eggs), larval weight (503 ± 0.02 mg), pupal weight (263 ± 0.01 mg) and adult female weight (128 ± 0.0mg) compared with natural hosts. Cotton was the least preferred host with a longer life cycle of 49.5 ± 0.50 days. Head capsule width and length were measured and the growth rate was validated using Dyar's rule. The mean width and length of the head capsule of first-instar larvae of FAW on different hosts was 0.35 ± 0.00 mm. The maximum width (2.76 ± 0.03 mm) and length (2.31 ± 0.03 mm) were observed in sixth-instar larvae grown on diet. The results of this study will be instrumental in understanding and formulating management strategies for FAW.

An experiment was conducted to test the efficacy of grease and emamectin benzoate in a randomized complete block design with five replications to reduce fall armyworm, Spodoptera frugiperda (J.E. Smith) larvae load from a maize field in the winter seasons of 2020 and 2021 in Sarlahi, Nepal. Standard agronomic package of practices followed for crop proper growth and development, and plant spacing maintained at 20 × 60 cm2 with a plot size of 390 m2. The treatments were applied when the maize crop was at knee height and larvae damaged nearly 5–8% of the total plants. Emamectin benzoate sprayed at 0.4 g/liter of water and grease of about 0.15 g applied to the maize whorl or tip of a drooping leaf that touched the soil. A significant reduction in larval infestation was observed after 7-days of treatment applications. Fall armyworm larvae were found dead in the chemical-sprayed plots, but they were absent in the grease-applied fields. No crop damage was observed among the grease-treated plants, which might be due to restrictions in the movement of larvae on the maize crop. The armyworm larvae might get irritated, feel insecure, and move far away from the test plots searching for food materials. Thus, an eco-friendly material like grease can be used as an agroecological method for managing fall armyworm larvae among small-scale land-holding maize farmers.

Developmental characteristics of larvae of corn carworm, Heliothis zea (Boddie); fall armyworm, Spodoptera frugiperda (J. E. Smith); and southwestern corn borer, Zeadiatraea grandiosella (Dyar), reared on sorghum, were compared with the same species reared on artificial diet. In each of the 3 species, no significant differences were observed in head-capsule widths in the several instars of larvae fed on the 2 diets. It was found, further, that a change in diet from sorghum to artificial diet or from artificial diet to sorghum, at various intervals in the larval cycle, had little effect on pupal weight or duration of the larval cycle of the 3 species.

The fall armyworm (FAW) Spodoptera frugiperda is a destructive and polyphagous pest of many essential food crops including maize and rice. The FAW is hard to manage, control, or eradicate, due to its polyphagous nature and voracity of feeding. Here, we report the characterization and functional analysis of the detoxification gene CYP302A1 and how S. frugieprda larvae use a detoxification mechanism to adapt host plants. Results demonstrated that CYP302A1 expression levels were much higher in midgut tissue and the older S. frugiperda larvae. Our current studies revealed the enhanced P450 activity in the midguts of S. frugiperda larvae after exposure to rice plants as compared to corn plants and an artificial diet. Furthermore, higher mortality was observed in PBO treated larvae followed by the exposure of rice plants as compared to the corn plant. The dsRNA-fed larvae showed downregulation of CYP302A1 gene in the midgut. At the same time, higher mortality, reduced larval weight and shorter developmental time was observed in the dsRNA-fed larvae followed by the exposure of rice plant as compared to the corn plant and DEPC-water treated plants as a control. These results concluded that the inducible P450 enzyme system and related genes could provide herbivores with an ecological opportunity to adapt to diverse host plants by utilizing secondary compounds present in their host plants.